Ligand site interchange

Fig. 13. Schematic representation of ligand-site exchange in 32 (1,2-shift, topomerization) through a bicapped-tetrahedral intermediate or transition state. Upper part (Ph,Cl) interchange lower part (0,0) interchange.45 Reproduced with permission from the American Chemical Society.

Ligand isomerism is observed in compounds that have the same stoichiometry and metal core, but in which two or more ligands formally interchange coordination sites, for example compounds E and F (Fig. 

A transposition is the interchange of ligands on two sites, leaving all other sites and ligands undisturbed. It iswell-known that any permutation can be expressed as a product of transpositions. A product of an odd (even) number of transpositions is called an odd (even) permutation. Although the expression of a given permutation as a product of transposition is not always unique, the odd or even character is the same for all such decompositions. 

However, even for members of class a, reflection and permutation are not necessarily equivalent. For example, interchange of (achiral) ligands at an equatorial and an apical site of a trigonal bipyramidal skeleton yields a diastereomer. The tetrahedral skeleton (with achiral ligands) is unique in showing this property. Thus, even given the importance of this skeleton, it would be unwise to base broadly applicable terms on observations which are only valid for tetrahedral models. 

Substitution at one or more metal sites will generally break the symmetry of the cluster core, and can greatly influence its electronic properties and reactivity. Consider, for example, the possible substitutions of a metal M into an octahedral core of composition MfiX v (x = 8, 12). The first substitution will afford an MsM X core, for which the symmetry has been lowered to C4v. A second substitution generates an M4M 2Xx core with two possible isomers One in which the M atoms are positioned at trans vertices (D4/,) and another where they are positioned at cis vertices (C2v). With a third substitution to give an M3M 3Xx core, fac and mer isomers become possible, while further substitutions simply repeat the pattern with M and M interchanged. Here again, the substitutions can be anticipated to alter the basic electronic properties of the cluster. Moreover, the outer-ligand substitution chemistry could potentially be quite different... 

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